1. Field of the Invention
The invention relates generally to a laser transfer method for the deposition of a rheological fluid or system onto a substrate.
2. Description of the Prior Art
A direct-write process is a technique which allows the creation of a pattern and the transfer of material simultaneously onto a given surface or substrate. To be most useful, it does not require any masks or pre-existing form and is usually done under ambient temperature and pressure conditions. Direct-write technologies have been developed in response to a need in the electronics industry for a means to rapidly prototype passive circuit elements on various substrates, especially in the mesoscopic regime, that is, electronic devices that straddle the size range between conventional microelectronics (sub-micron-range) and traditional surface mount components (10+ mm-range). (Direct-writing may also be accomplished in the sub-micron range using electron beams or focused ion beams, but these techniques, because of their small scale and vacuum requirements, are not appropriate for large-scale rapid prototyping.) Direct-writing allows for circuits to be prototyped without iterations in photolithographic mask design and allows the rapid evaluation of the performance of circuits too difficult to accurately model. Further, direct-writing allows for the size of printed circuit boards and other structures to be reduced by allowing passive circuit elements to be conformally incorporated into the structure. Direct-writing can be controlled with CAD/CAM programs, thereby allowing electronic circuits to be fabricated by machinery operated by unskilled personnel or allowing designers to move quickly from a design to a working prototype. Mesoscopic direct-write technologies have the potential to enable new capabilities to produce next generation applications in the mesoscopic regime. Other applications of direct-write technologies in microelectronic fabrication include forming ohmic contacts, forming interconnects for circuit and photolithographic mask repair, device restructuring and customization, design and fault correction.
Currently known direct-write technologies for adding materials to a substrate include ink jet printing, Micropen, laser induced forward transfer (LIFT), laser chemical vapor deposition (LCVD), laser particle guidance (Optomec, Inc.), and laser engineered nano-shaping (LENS). Currently known direct-write technologies for removing material from a substrate include laser machining, laser trimming, and laser drilling.
The direct-writing techniques of ink jet printing, screen-printing, and Micropen are wet techniques, that is, the material to be deposited is combined with a solvent or binder and is applied onto a substrate. In the case of ink jet printing, inks with very low viscosity are required so that they can be forced through nozzles via mechanical or thermal forces. In the case of screen-printing and Micropen, inks with relatively high viscosities are required so as to minimize their spreading once applied to the substrate. The solvent or binder must later be removed by a drying or curing process, which limits the flexibility and capability of these approaches. In all these techniques, only inks within a narrow range of viscosity can be used and therefore the choices of materials or formulations that can be transferred are rather limited.
In the direct-writing technique known as xe2x80x9claser induced forward transferxe2x80x9d (LIFT), a pulsed laser beam is directed through a laser-transparent target substrate to strike a film of material coated on the opposite side of the target substrate. The laser vaporizes the film material as it absorbs the laser radiation and, due to the transfer of momentum, the material is removed from the target substrate and is redeposited on a receiving substrate that is placed in proximity to the target substrate. Laser induced forward transfer is typically used to transfer opaque thin films, typically metals, from a pre-coated laser transparent support, typically glass, SiO2, Al2O3, SrTiO3, etc., to the receiving substrate. Various methods of laser-induced forward transfer are described in, for example, the following U.S. patents and publications incorporated herein by reference: U.S. Pat. No. 4,752,455 to Mayer, U.S. Pat. No. 4,895,735 to Cook, U.S. Pat. No. 5,725,706 to Thoma et al., U.S. Pat. No. 5,292,559 to Joyce, Jr. et al., U.S. Pat. No. 5,492,861 to Opower, U.S. Pat. No. 5,725,914 to Opower, U.S. Pat. No. 5,736,464 to Opower, U.S. Pat. No. 4,970,196 to Kim et al., U.S. Pat. No. 5,173,441 to Yu et al., and Bohandy et al., xe2x80x9cMetal Deposition from a Supported Metal Film Using an Excimer Laser, J. Appl. Phys. 60 (4) Aug. 15, 1986, pp 1538-1539. Because the film material is vaporized by the action of the laser, laser induced forward transfer is inherently a pyrolytic technique used to deposit simple one-component materials and typically cannot be used to deposit complex crystalline, multi-component materials as they tend to decompose when vaporized and may become amorphous upon condensation. Moreover, because the material to be transferred is vaporized, it becomes more reactive and can more easily become degraded, oxidized, or contaminated. The method is not well suited for the transfer of organic materials, since many organic materials are fragile, thermally labile, and can be irreversibly damaged during deposition. Moreover, functional groups on an organic polymer can be irreversibly damaged by direct exposure to laser energy. Other disadvantages of the laser induced forward transfer technique include poor uniformity, morphology, adhesion, and resolution. Further, because of the high temperatures involved in the process, there is a danger of ablation or sputtering of the support, which can cause the incorporation of impurities in the material that is deposited onto the receiving substrate. Another disadvantage of laser induced forward transfer is that it typically requires that the coating of the material to be transferred be a thin coating, generally less than 1 xcexcm thick. Because of this requirement, it is very time-consuming to transfer large amounts of material. Finally, LIFT was not designed originally for the transfer of rheological systems. The art of applying a solid coating to the target substrate was already well established in the field and a rheological coating as described in this invention would have added extra complexity to its manufacture, use, and storage.
In a simple variation of laser induced forward transfer, the target substrate is coated with several layers of materials. The outermost layer, that is, the layer closest to the receiving substrate, consists of the material to be deposited and the innermost layer consists of a material that absorbs laser energy and becomes vaporized, causing the outermost layer to be propelled against the receiving substrate. Variations of this technique are described in, for example, the following U.S. patents and publications incorporated herein by reference: U.S. Pat. No. 5,171,650 to Ellis et al., U.S. Pat. No. 5,256,506 to Ellis et al., U.S. Pat. No. 4,987,006 to Williams et al., U.S. Pat. No. 5,156,938 to Foley et al. and Tolbert et al., xe2x80x9cLaser Ablation Transfer Imaging Using Picosecond Optical pulses: Ultra-High Speed, Lower Threshold and High Resolutionxe2x80x9d Journal of Imaging Science and Technology, Vol.37, No.5, Sept./Oct. 1993 pp.485-489. A disadvantage of this method is that, because of the multiple layers, it is difficult or impossible to achieve the high degree of homogeneity of deposited material on the receiving substrate required, for example, in the construction of electronic devices, sensing devices or passivation coatings. In addition, the multiple layers tend to leave residues that may contaminate the transferred material or degrade its properties.
The direct-write technique called laser chemical vapor deposition (LCVD) utilizes a laser beam focused on the surface of a substrate to induce localized chemical reactions. Usually the surface of the substrate is coated with a metal-organic precursor, which is either pyrolyzed or photolyzed locally where the laser beam scans. Pyrolytic LCVD involves essentially the same mechanism and chemistry as conventional thermal LCVD, and it has found major use in direct-writing of thin films for semiconductor applications. In photolytic LCVD, the chemical reaction is induced by the interaction between the laser light and the precursors. A limitation of both processes is that they must be carried out under controlled atmospheres such as inside a vacuum system, and they tend to exhibit slow deposition rates. In addition they are not well suited for direct-write applications where multilayers of dissimilar types of materials need to be produced.
The direct-write technique called laser engineered nano-shaping (LENS) utilizes a laser beam to melt powders of various materials as they come in contact with the substrate surface. LENS is a process that works well for making thick layers of metals. However, the high melting points exhibited by most ceramics required the use of high power laser beams, which cause the overheating of the substrate and surrounding layers. Furthermore, many ceramics once melted will not exhibit their original crystalline structure after solidification. In addition, because the materials being deposited must first melt and then resolidify, the layers are under large stresses, which cause their delamination.
All these techniques involve the laser transfer of matter that is not deliberately subject to any type of deformation or flow. Rather, the matter is subject either to changes in phase, i.e. solid to vapor or to change in composition, i.e. decomposition of a matrix. At present there is no record of the use of lasers to forward transfer rheological systems, i.e. fluids, gels, or pastes, taking advantage of the rheological properties, for the purpose of laser direct-write applications.
The wet techniques described above cannot make a pattern with a resolution on the order of a few microns. The laser transfer techniques described above cannot be used with a rheological fluid. There remains a need for a laser transfer method that can produce a pattern of a rheological fluid with a resolution on the order of a few microns.
U.S. Pat. No. 6,177,151 to Chrisey et al. discloses the MAPLE-DW (Matrix Assisted Pulsed Laser Evaporation Direct-Write) method and apparatus. The method comprises the use of laser energy to cause a composite material to volatilize, desorb from a laser-transparent support, and be deposited on a receiving substrate. The composite material comprises a matrix material and a transfer material. The transfer material is the material desired to be transferred to the receiving substrate. The matrix material is more volatile than the transfer material and binds the transfer material into the composite material. The laser energy causes the matrix material to volatilize and propel the transfer material onto the receiving substrate. The properties of the transfer material are preserved after deposition. This method will be further described in the Detailed Description of the Preferred Embodiments below. U.S. Pat. No. 6,177,151 is primarily directed to the transfer of solid composite materials.
In the prior use of MAPLE-DW, the materials of interest are contained in a solid matrix coating on one side of a target substrate. The laser pulse impacts this layer and transfers the materials of interest to an acceptor substrate while decomposing the volatile components. Due to the solid form of this matrix coating, there is less densification and increased spatial scatter of the material after the transfer process. The loosely consolidated nature of the transferred material also results in poor adhesion to the substrate and poor intrinsic mechanical strength as well. A rheological fluid could result in transfers with better spatial resolutions, less debris, smoother and more uniform cross-sectional profile, higher density, improved functional properties, and adhesion to the substrate.
U.S. patent application Ser. No. 10/068,315 filed on Feb. 8, 2002 by Ringeisen et al., also a continuation-in-part application of U.S. patent application Ser. No. 09/671,166, discloses the use of MAPLE-DW where the matrix material is a rheological fluid, but only when the transfer material is living or active biomaterial.
It is an object of the invention to provide methods for depositing a rheological fluid on a receiving substrate using a laser forward transfer apparatus that can produce a pattern with a resolution on the order of a few microns.
It is a further object of the invention that the method use laser fluences lower than that required by other laser transfer methods.
It is a further object of the invention that the method allow for higher density and linewidth definition in the transferred material.
These and other objects of the invention are accomplished by a method for laser deposition comprising the steps of: providing a receiving substrate; providing a target substrate; wherein the target substrate comprises a laser-transparent support coated with a coating on a surface facing the receiving substrate; and exposing the coating to laser energy through the laser-transparent support at a defined target location comprising a rheological fluid to evaporate a portion of the rheological fluid adjacent to the laser-transparent support at the defined target location; wherein the non-evaporated rheological fluid at the defined target location is propelled by the evaporated rheological fluid away from the laser-transparent support and toward the receiving substrate; and wherein the non-evaporated rheological fluid is deposited at a defined receiving location on the receiving substrate to form a deposit.